Perspective
If you stand between two rails of a railway line and you look in the distance then it seems that they are getting closer together. And at last they disappear at one point on the horizon. You can also draw this. While the rails lay flat on the ground, the lines on the drawing from left and right to the middle go up. You call this perspective. Characteristic of perspective is always a disappearance point. In the distance you can not see the rails anymore, they have disappeared. In your drawing they also come together and see
you no longer have them If you're looking for a random painting in a museum, you can draw imaginary lines along the displayed objects and all those lines come together in the disappearance.